Glass fiber reinforced plastics exhibit superior physical and mechanical properties over unreinforced plastics. As a result, they are widely used in the automotive industry to produce a wide variety of interior and exterior parts. One of the most frequently used glass fiber reinforced plastic systems is sheet molding compound (SMC). SMC is a material composed of reinforcing chopped glass fibers combined with a complex resin system formulated in an almost infinite number of ways. A typical formulation includes ingredients such as inorganic fillers, thickeners, curing agents, and internal mold release agent in addition to a carrier polyester resin and a shrinkage-control agent.
The flexibility in SMC formulation lends itself to a variety of end use properties for molded parts which are suitable for a wide spectrum of commercial applications. One major application is in the automotive industry for producing exterior body panels. For many years, SMC parts have been used in exterior body parts such as hoods, deck lids, and door panels. The application of SMC materials in automotive exterior body parts is sometimes limited by appearance problems caused by inadequate processing and product design. Some of these commonly observed appearance problems are sink marks over ribs and bosses, waviness of the surface, and surface porosity. Some of these problems have been solved by either making a modification in the formulation or by adding an extra step in the processing of SMC parts. For instance, the use of a low profile agent, or shrinkage control agent, have been found to greatly improve the surface waviness on molded SMC panels. The process of molded coating by injecting a thin layer of specially formulated coating into an SMC mold immediately after the completion of the SMC cure cycle is a technique found to be effective in producing a surface free from porosity.
Although the utilization of the molded coating and the shrinkage control agent has reduced the extent of sink marks, traces of sink marks still exist on SMC panels having integral ribs and bosses. One way of solving this problem is to style and design surface contours such that the small traces of sink marks are concealed. However, for large flat horizontal panels such as automobile hoods and deck lids which require numerous ribs to achieve the required stiffness, sink marks cannot always be concealed with design lines. As a result, in order to avoid sink marks in the manufacturing of many large horizontal parts, a design of two piece panels bonded together are frequently used. In a bonded two piece panel design, a flat panel having a perfect appearance surface is bonded by adhesives to a support panel having ribs for stiffness and bosses for mounting hardware. This is a very costly design in that not only two separate pieces of panels need to be molded in separate processes, but also one additional bonding step must be used to bond the two panels together. It is therefore more desirable to design a one piece panel having reinforcing means such as ribs and bosses on the back and a perfect appearance surface in the front that can be processed in one molding step.
It is, therefore, an object of the present invention to provide a method of producing chopped glass fiber reinforced plastic panels having integral ribs and bosses and an appearance surface without sink mark defects.
It is another object of the present invention to provide a method of producing glass fiber reinforced plastic panels having integral ribs and bosses and an appearance surface without sink marks defects by balancing the molding pressure in the ribs and bosses with the molding pressure in the flat panel portion.